A US indictment of WikiLeaks founder Julian Assange under the 93-year-old Espionage Act is imminent, his lawyer said yesterday.

But prosecuting Assange would be unconstitutional, because it violates his free-speech protections “and puts at risk all media organizations in the US,” attorney Jennifer Robinson told ABC News.

Attorney General Eric Holder said Tuesday he had launched a criminal investigation of WikiLeaks because its “arrogant, misguided” actions put Americans at risk.

US prosecutors could be thwarted if Assange, now held in Britain, is extradited to Sweden to face rape charges.

Swedish officials have already signaled their unwillingness to extradite Assange to the United States.

Several major firms, including MasterCard, Visa and PayPal, were cyber-attacked last week because hackers accused them of cooperating with US authorities in trying to crush WikiLeaks for releasing 250,000 secret cables.

Holder denied yesterday that Washington is leaning on companies to stop working with WikiLeaks. “We have not pressured anybody,” he told reporters in San Francisco.